Electrowetting on dielectric (EWOD) is a well known technique for manipulating droplets of fluid by application of an electric field. It is thus a candidate technology for digital microfluidics for lab-on-a-chip technology. An introduction to the basic principles of the technology can be found in “Digital microfluidics: is a true lab-on-a-chip possible?”, R. B. Fair, Microfluid Nanofluid (2007) 3:245-281).
FIG. 1 shows a part of a conventional EWOD device in cross section. The device includes a lower substrate 72, the uppermost layer of which is formed from a conductive material which is patterned so that a plurality of electrodes 38 (e.g., 38A and 38B in FIG. 1) are realized. The plurality of electrodes may be termed the EW drive elements. The droplet 4, consisting of a polar material (which is commonly also ionic), and is constrained in a plane between the lower substrate 72 and a top substrate 36. A suitable gap between the two substrates may be realized by means of a spacer 32, and a non-polar fluid 34 (e.g. oil) may be used to occupy the volume not occupied by the liquid droplet 4. An insulator layer 20 disposed upon the lower substrate 72 separates the conductive electrodes 38A, 38B from a first hydrophobic surface 16 upon which the liquid droplet 4 sits with a contact angle 6 represented by θ. On the top substrate 36 is a second hydrophobic layer 26 with which the liquid droplet 4 may come into contact. Interposed between the top substrate 36 and the second hydrophobic layer 26 is a top substrate electrode 28.
The contact angle θ6 is defined as shown in FIG. 1, and is determined by the balancing of the surface tension components between the solid-liquid (γSL), liquid-gas (γLG) and non-ionic fluid (γSG) interfaces, and in the case where no voltages are applied satisfies Young's law, the equation being given by:
                              cos          ⁢                                          ⁢          θ                =                                            γ              SG                        -                          γ              SL                                            γ            LG                                              (                  equation          ⁢                                          ⁢          1                )            In certain cases, the relative surface tensions of the materials involved (i.e the values of γSL, γLG and γSG) may be such that the right hand side of equation (1) is less than −1. This may commonly occur in the case in which the non-ionic fluid 34 is oil. Under these conditions, the liquid droplet 4 may lose contact with the hydrophobic surfaces 16 and 26, and a thin layer of the non-polar fluid 34 (oil) may be formed between the liquid droplet 4 and the hydrophobic surfaces 16 and 26.
In operation, voltages termed the EW drive voltages, (e.g. VT, V0 and V00 in FIG. 1) may be externally applied to different electrodes (e.g. drive element electrodes 28, 38A and 38B, respectively). The resulting electrical forces that are set up effectively control the hydrophobicity of the hydrophobic surface 16. By arranging for different EW drive voltages (e.g. V0 and V00) to be applied to different drive element electrodes (e.g. 38A and 38B), the liquid droplet 4 may be moved in the lateral plane between the two substrates 72 and 36.
U.S. Pat. No. 6,565,727 (Shenderov, issued May 20, 2003) discloses a passive matrix EWOD device for moving droplets through an array.
U.S. Pat. No. 6,911,132 (Pamula et al., issued Jun. 28, 2005) discloses a two dimensional EWOD array to control the position and movement of droplets in two dimensions.
U.S. Pat. No. 6,565,727 further discloses methods for other droplet operations including the splitting and merging of droplets, and the mixing together of droplets of different materials.
U.S. Pat. No. 7,163,612 (Sterling et al., issued Jan. 16, 2007) describes how TFT based electronics may be used to control the addressing of voltage pulses to an EWOD array by using circuit arrangements very similar to those employed in AM display technologies.
The approach of U.S. Pat. No. 7,163,612 may be termed “Active Matrix Electrowetting on Dielectric” (AM-EWOD). There are several advantages in using TFT based electronics to control an EWOD array, namely:                Driver circuits can be integrated onto the AM-EWOD array substrate.        TFT-based electronics are well suited to the AM-EWOD application. They are cheap to produce so that relatively large substrate areas can be produced at relatively low cost.        TFTs fabricated in standard processes can be designed to operate at much higher voltages than transistors fabricated in standard CMOS processes. This is significant since many EWOD technologies require EWOD actuation voltages in excess of 20V to be applied.        
A disadvantage of U.S. Pat. No. 7,163,612 is that it does not disclose any circuit embodiments for realizing the TFT backplane of the AM-EWOD.
EP2404675 (Hadwen et al., published Jan. 11, 2012) describes array element circuits for an AM-EWOD device. Various methods are known for programming and applying an EWOD actuation voltage to the EWOD drive electrode. The voltage write function described includes a memory element of standard means, for example, based on Dynamic RAM (DRAM) or Static RAM (SRAM) and input lines for programming the array element.
U.S. Pat. No. 8,173,000 (Hadwen et al., issued May 8, 2012) describes an AM-EWOD device with array element circuit and method for writing an AC actuation voltage to the electrode. The AC drive scheme described by this patent utilizes the application of AC signals to both the drive element electrode and top substrate electrodes of the device. Therefore, the device is capable of generating a voltage difference between the electrodes that varies between +VEW and −VEW, whilst the transistors in the array element circuit are only ever required to operate with a rail-to-rail voltage of VEW. This patent further describes methods of driving the device sometimes in an AC and sometimes in a DC mode, so as to be compatible with the operation of integrated sensor functions.
US application 2012/0007608 (Hadwen et al., published Jan. 12, 2012) describes how an impedance (capacitance) sensing function can be incorporated into the array element. The impedance sensor may be used for determining the presence and size of liquid droplets present at each electrode in the array.
US application US2011/0180571 (Srinivasan et al., published Jul. 28, 2011) describes how using adjustable electrowetting voltages may help to maintain the stability of the oil film that is formed between the liquid droplet 4 and the hydrophobic surfaces 16 and 26. They describe how the maintenance of the oil film between the droplet and the surface of the droplet actuator is an important factor in optimum operation of the droplet actuator. A stabilized oil film leads to less contamination, such as contamination due to absorption and resorption. In addition, maintenance of the oil film provides for more direct electrowetting and allows for the use of lower voltages for droplet operations. They further describe how different voltages may be used for performing different operations, for example a higher voltage may be used in order to elute a droplet from a reservoir than as would be used to move a droplet between adjacent array elements.